With the change of state from wakefulness to sleep, muscle activity and tone decrease, and they are lost completely during rapid eye movement (REM) sleep. Therefore, one would anticipate few, if any, movement problems associated with sleep. In spite of these physiologic changes, however, normal sleep is not totally free of movement. The average sleeper moves about 40 to 50 times a night and
this number changes in certain situations.
For example, sleep deprivation
results in a sleep with fewer movements.1

In addition to total body movements,
limb jerks and twitches also occur in
normal sleepers. They typically occur
during sleep onset (sleep starts or hypnic
jerks) or in association with REM
sleep.2 Such movements in sleep are
increased in persons who have movement
disorders while awake, such as
those with Parkinson disease (PD) or
Tourette syndrome (TS).

When considering disorders of
movement during sleep, the physician
should ascertain whether abnormal
movements also occur during awake
periods. Abnormal movements that are
present during the day, such as the motor
disturbance of PD or TS, are usually
quiescent during sleep, while those occurring
primarily in sleep (eg, nocturnal
epilepsies, parasomnias, restless legs
syndrome [RLS], periodic limb movements
of sleep [PLMS]) rarely intrude
into awake periods. Some disorders,
such as seizures, manifest predominantly
during sleep, but may occasionally
occur during periods of
wakefulness. A classification of sleeprelated
movement disorders is presented
in Table 1. I will discuss the more salient
of these disorders in this article.

SLEEP-SPECIFIC MOVEMENT DISORDERSPeriodic limb movements of sleep

Previously referred to as nocturnal
myoclonus, PLMS are regarded as a
distinct nosologic entity, even though
they overlap a great deal with RLS and
other sleep disorders. These movements
primarily occur in the lower limbs and
are classically described as phasic extensions
of the big toe and dorsiflexion at
the ankle, occurring with a periodicity
of 20 to 40 seconds. Flexion at the knee
and hip may occur, and movements may involve the upper limbs. Both lower
limbs are usually involved but not necessarily
symmetrically or simultaneously.
Sometimes only one leg is involved, or
the phenomenon may alternate from one
leg to the other.3 The electromyographic
characteristics of the movements are
varied and are usually of longer duration
than those of classic myoclonus,
typically 1.5 to 2.5 seconds long (range,
0.5 to 5 seconds). There may be an initial
myoclonic jerk followed by a tonic
contraction, or a polyclonic contraction
with or without a tonic component.

PLMS are common in healthy elderly persons, with 45% of 65- to 76-year olds,
women more often than men,
having 5 PLMS per hour at night4; the
condition is rare before the age of 30.
PLMS occur in a number of sleep disorders,
particularly RLS, but also
narcolepsy, REM sleep behavior disorder
(RSBD), and obstructive sleep apnea.
PLMS also occur in awake subjects
with RLS but only rarely in controls.5

The clinical significance of PLMS
continues to be debated, since many
studies have failed to demonstrate an
association between PLMS and symptoms
of sleep disturbance.6,7 It is possible
that people who complain of
insomnia caused by leg movements
may have a lower threshold of arousal.

The pathogenesis of PLMS is not
clear. Lesion, imaging, and laboratory
studies indicate neuronal hyperexcitability
with involvement of brainstem
and spinal cord structures, in particular,
the central pattern generator for gait.8 There is also evidence of decreased dopaminergic
transmission.9 PLMS have also
been associated with neuroleptic-induced
akathisia.10 Tricyclic and selective serotonin
reuptake inhibitor antidepressants
may induce or worsen PLMS,11 presumably
via serotonergic influences on dopaminergic
transmission.